The basic principle of the conventional combustion engine is the adiabatic expansion of a compressed and heated gas in such a manner that part of the energy of the gas is converted into mechanical work. The simplest engine based on this principle is a cylinder with one closed and one open end and a sliding piston in it that is pushed by the expanding gas towards the open end. The motion of the piston is transmitted to a crankshaft by means of a connecting rod. The literature on combustion engines mentions no other principle than the adiabatic expansion principle because none other has been used in the recent 150 years. To a person familiar with the theory and the practice of the combustion engine it will, therefore, appear that no other type of expansion can be used in a combustion engine. The description of the invention must therefore connect to the conventional engine as the point of reference.
The state of a gas is always in agreement with the gas law EQU pv=NRT
where p is the pressure, v the volume, N the quantity in mols, R the universal gas constant, and T the absolute temperature. Among the four variables p, v, N, and T, any 2, 3 or 4 may vary in a change of state. The textbooks on thermodynamics that treat the general subject usually assume that N is constant during the change of state. On this condition adiabatic expansion is defined as a change at constant entropy ##EQU1## or as a change of state without exchange of heat with the environment. Isothermal expansion is then defined as a change of state at constant temperature T EQU pv=NRT=const.
According to the invention, the change of state occurs at constant pressure p and constant temperature T ##EQU2##
Textbooks that give the gas law for one mole, N=1, of the gas may give the impression that expansion at constant T and constant p is not possible because then, at a constant N=1, v would also be constant, i.e. there would be no change.
The conventional engine operates in cycles of four strokes. A stroke is the displacement of the piston from one extreme position, x.sub.1 to the other extreme position x.sub.2 with x.sub.2 &gt;x.sub.1. The cycle is terminated when the spent gas has been exhausted, and the piston is in the position x.sub.1. The first stroke of the next cycle sucks air from outside into the cylinder and mixes it with injected gaseous fuel. The second stroke compresses this gas. In the position x.sub.1, at the beginning of the third stroke, the fuel is ignited. Then follows the third stroke that delivers work to the crankshaft. The fourth stroke exhausts the spent gas. FIG. 1, a graph from a common Engineer's handbook shows p against v for the three strokes following upon the first, suction, stroke. The abscissa .epsilon. is the compression ratio. Two scales are marked on the ordinate, the absolute pressure and the pressure above atmospheric. The smaller loop represents a rapidly burning fuel that is ignited at .epsilon.=0.2,p=9. The pressure increases from the heat supplied almost vertically to p=26. Then follows the expansion under delivery of work to the crankshaft. The larger loop represents a slowly burning fuel, the heating curve is in this case almost horizontal. The more slowly the fuel burn, the more the adiabatic expansion approaches isothermal expansion. For the adiabatic expansion EQU pv.sup..kappa. =const.
with .kappa.=1.4 for a diatomic gas, such as air. For the isothermal expansion .kappa.=1. The result is a polytropic expansion with 1&lt;.kappa.&lt;1.4. The heat loss through the wall of the cylinder has an effect in the same direction. As a consequence the curves of p against v for the compression and for the expansion in FIG. 1 are polytropic. Clearly, the more isothermal the expansion is, the smaller is the temperature drop during the expansion. The work done in the adiabatic expansion is ##EQU3## where indices 1 and 2 denote start and end of the expansion. Thus, for the adiabatic expansion, .kappa.=1.4., the value of W increases with the temperature drop during the expansion. For the isothermal expansion .kappa.=1 and T.sub.2 =T.sub.1 and W is equal to the heat supplied during the expansion, but T.sub.2 and P.sub.2 are much larger than for the adiabatic expansion. Much more energy is exhausted with the spent gas in isothermal expansion than in adiabatic expansion. The net result is that the advantages of the isothermal expansion are worthless unless the spent gas is recovered and used again in the following cycle.
At the end of the cycle in FIG. 1, P.sub.2 and T.sub.2 are much greater than at the start of the cycle. In order to use the exhaust gas in a following cycle, at the same compression ratio, it would be necessary to reduce its pressure and its temperature. But then, of course, so much less of its energy would be recovered. After compression and heating the gas shown in FIG. 1 is so hot and gives off so much heat to the wall, that the engine would be ruined in a very few cycles unless the wall is very effectively cooled.
In the case of the isothermal expansion, T is the same at the end of the stroke as at the start of the stroke, but p is reduced proportionally to the expansion ratio. The pressure could be restored by compression, but then the temperature would increase correspondingly unless, of course, the compression is isothermal. This means that all the work done in the preceding stroke would be converted into heat in the coolant.
The conventional engine receives work and heat and delivers more work and less heat. It converts heat to work and also work to heat. The net amount of heat converted to work is represented by the area inside the closed loop in FIG. 1. Compression is reversible. The work received as work of compression in the stroke of compression is delivered as work of expansion in the following stroke of expansion. At the transition from the former stroke to the latter the engine received heat from the burning fuel. The reversibility is not perfect, however. The compression raises the temperature of the compressed gas and, therefore, the loss of heat through the wall of the cylinder. The reason for the compression is that it is a means to controlling the rate and the completion of the combustion of the fuel. These considerations are not so important now as they used to be because in recent years equipment has become available that allows the complete control of the outside combustion and the injection of the hot products into the engine in determined amounts and at determined intervals and sequences.